1. Field of the Invention
The invention relates to an imaging system for imaging an object field arranged in an object surface of the imaging system onto an image field arranged in an imaging surface of the imaging system while creating at least one intermediate image. In a preferred field of application the imaging system is designed as a catadioptric projection objective for a microlithographic projection exposure system designed for projection using radiation in the ultraviolet spectrum or as an integral part thereof.
2. Description of the Related Art
Catadioptric projection objectives are, for example, employed in projection exposure systems, in particular wafer scanners or wafer steppers, used for fabricating semiconductor devices and other types of microdevices and serve to project patterns on photomasks or reticles, hereinafter referred to generically as “masks” or “reticles,” onto an object having a photosensitive coating with ultrahigh resolution on a reduced scale.
In order create even finer structures, it is sought to both increase the image-end numerical aperture (NA) of the projection objective to be involved and employ shorter wavelengths, preferably ultraviolet light with wavelengths less than about 260 nm. However, there are very few materials, in particular, synthetic quartz glass and crystalline fluorides, that are sufficiently transparent in that wavelength region available for fabricating the optical elements required. Since the Abbe numbers of those materials that are available lie rather close to one another, it is difficult to provide purely refractive systems that are sufficiently well color-corrected (corrected for chromatic aberrations).
The high prices of the materials involved and limited availability of crystalline calcium fluoride in sizes large enough for fabricating large lenses represent problems, particularly in the field of microlithography at 157 nm for very large numerical apertures, NA, of, for example, NA=0.80 and larger. Measures that will allow reducing the number and sizes of lenses employed and simultaneously contribute to maintaining, or even improving, imaging fidelity are thus desired.
In optical lithography, high resolution and good correction status have to be obtained for a relatively large, virtually planar image field. It has been pointed out that the most difficult requirement that one can ask of any optical design is that it have a flat image, especially if it is an all-refractive design. Providing a flat image requires opposing lens powers and that leads to stronger lenses, more system length, larger system glass mass, and larger higher-order image aberrations that result from the stronger lens curvatures. Conventional means for flattening the image field, i.e. for correctings the Petzval sum in projection objectives for microlithography are discussed in the article “New lenses for microlithography” by E. Glatzel, SPIE Vol. 237 (1980), pp. 310-320.
Concave mirrors in optical imaging systems have been used for some time to help solve problems of color correction and image flattening. A concave mirror has positive power, like a positive lens, but the opposite sign of Petzval curvature. Also, concave mirrors do not introduce color problems.
In view of the aforementioned problems, catadioptric systems that combine refracting and reflecting elements, i.e., in particular, lenses and concave mirrors, are primarily employed for configuring high-resolution projection objectives of the aforementioned type.
Unfortunately, a concave mirror is difficult to integrate into an optical design, since it sends the radiation right back in the direction it came from. Intelligent designs integrating concave mirrors without causing mechanical problems or problems due to beam vignetting or obscuration are desirable.
Catadioptric projection objectives having one common straight optical axis and at least two concave mirrors have been proposed to provide systems with good color correction and moderate lens mass requirements. The U.S. Pat. No. 6,600,608 B1 discloses a catadioptric projection objective having a first, purely refractive objective part for imaging a pattern arranged in the object plane of the projection objective into a first intermediate image, a second objective part for imaging the first intermediate image into a second intermediate image and a third objective part for imaging the second intermediate image directly, that is without a further intermediate image, onto the image plane. The second objective part is a catadioptric objective part having a first concave mirror with a central bore and a second concave mirror with a central bore, the concave mirrors having the mirror faces facing each other and defining an intermirror space or catadioptric cavity in between. The first intermediate image is formed within the central bore of the concave mirror next to the object plane, whereas the second intermediate image is formed within the central bore of the concave mirror next to the object plane. The objective has axial symmetry and a field centered around the optical axis and provides good color correction axially and laterally. However, since the reflecting areas of the concave mirrors exposed to the radiation are interrupted at the bores, the pupil of the system is obscured.
The Patent EP 1 069 448 B1 discloses catadioptric projection objectives having two concave mirrors facing each other and an off-axis object and image field. The concave mirrors are part of a first catadioptric objective part imaging the object onto an intermediate image positioned adjacent to a concave mirror. This is the only intermediate image, which is imaged to the image plane by a second, purely refractive objective part. The object as well as the image of the catadioptric imaging system are positioned outside the intermirror space defined by the mirrors facing each other. Similar systems having two concave mirrors, a common straight optical axis and one intermediate image formed by a catadioptric imaging system and positioned besides one of the concave mirrors are disclosed in Japanese patent application JP 2002208551 A and US patent application US 2002/0024741.
US patent application US 2004/0130806 (corresponding to European patent application EP 1 336 887) discloses catadioptric projection objectives having off-axis object and image field, one common straight optical axis and, in that sequence, a first catadioptric objective part for creating a first intermediate image, a second catadioptric objective part for creating a second intermediate image from the first intermediate image, and a refractive third catadioptric objective part forming the image from the second intermediate image. Each catadioptric system has two concave mirrors facing each other. The intermediate images lie outside the intermirror spaces defined by the concave mirrors.
Japanese patent application JP 2003114387 A and international patent application WO 01/55767 A disclose catadioptric projection objectives with off-axis object and image field having one common straight optical axis, a first catadioptric objective part for forming an intermediate image and a second catadioptric objective part for imaging the intermediate image onto the image plane of this system. Concave and convex mirrors are used in combination.
US patent application US 2003/0234992 A1 discloses catadioptric projection objectives with off-axis object and image field having one common straight optical axis, a first catadioptric objective part for forming an intermediate image, and a second catadioptric objective part for imaging the intermediate image onto the image plane. In each catadioptric objective part concave and convex mirrors are used in combination with one single lens.
U.S. provisional application with Ser. No. 60/536,248 filed on Jan. 14, 2004 by the applicant and published from non-provisional application Ser. No. 11/035,103 on Sep. 1, 2005 as US 2005/0190435A1 discloses a catadioptric projection objective having very high NA and suitable for immersion lithography at NA>1. The projection objective comprises: a first objective part for imaging the pattern provided in the object plane into a first intermediate image, a second objective part for imaging the first intermediate imaging into a second intermediate image, and a third objective part for imaging the second intermediate imaging directly onto the image plane. The second objective part includes a first concave mirror having a first continuous mirror surface and a second concave mirror having a second continuous mirror surface, the concave mirror faces facing each other and defining an intermirror space. At least the first intermediate image, preferably both intermediate images, are located geometrically within the intermirror space. The system has potential for very high numerical apertures at moderate lens mass consumption. The full disclosure of this document is incorporated into this application by reference.